Method of refining the structure of superplastic zinc base alloys



United States Patent 3,513,037 METHOD OF REFINING THE STRUCTURE OF SUPERPLASTIC ZINC BASE ALLOYS Cedric Charles Edward Colley, Wheatley, England, as-

signor to Pressed Steel Fisher Limited, Cowley, Oxford, England, a corporation of Great Britain No Drawing. Filed Jan. 19, 1968, Ser. No. 699,041 Claims priority, application Great Britain, Feb. 1, 1967,

Int. ci. czzr 1/16 U.S. Cl. 148-13 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method of refining the structure of those eutectoid or near eutectoid alloys that immediately after quenching to ambient temperature retain the high temperature solid phase state during an incubation period prior to the start of a transformation period wherein the alloy transforms to the eutectoid or partly eutectoid state.

The object of the invention is to provide a method of refining the structure of such alloys and thereby improving their superplasticity.

According to the invention a method of refining the structure of those eutectoid or near eutectoid alloys that immediately after quenching to ambient temperature retain the high temperature solid phase state during an incubation period prior to the start'of a transformation period wherein the alloy transforms to the eutectoid or partly eutectoid state, includes heating the alloy above its eutectoid teinperature, rapidly cooling the alloy to a temperature below ambient, and before or during the transformation period warming the alloy to at least ambient temperature.

Preferably warming begins just before the start of the transformation period so that entire transformation is accelerated.

Embodiments of the invention will now be described solely by way of example.

EXAMPLE I A specimen of zinc-aluminum alloy containing 78% by weight of zinc was heated for thirty minutes in a furnace maintained at 350 C. so that transformation into the high temperature a phase was complete. The specimen ice I was then quenched into a mixture of equal parts by volume of water and ethylene glycol maintained at 10 C.

The specimen was maintained at -10 C. for one hour, and then removed and placed in warm Water at 40 C. to raise the temperature quickly to 40 C. so that rapid transformation to the eutectoid state occurred.

Tensile tests at 250 C. and a strain rate of 1 inch/inch/ minute showed that the maximum fiow stress was 1950 p.s.i. compared with 2,300 p.s.i. for a control specimen which had been quenched to the ambient temperature only. Examination of the specimen with an electron microscope showed that the structure was finer than that of the control specimen.

EXAMPLE II A specimen of zinc-aluminium alloy containing by weight of zinc was heated for 30 minutes in a furnace maintained at 350 C. so that transformation into the a phase was complete. The specimen was then quenched in cold water and further cooled, in a mixture of solid carbon dioxide and methyl alcohol, to 20 C., remaining at the temperature for two hours. The specimen was removed and placed in Warm water to raise the temperature quickly to 25 C., so that rapid transformation to the a-l-B eutectoid state occurred.

Examination of the specimen with an electron microscope showed that the structure was finer than that of a comparative control specimen which had been quenched to ambient temperature and allowed to incubate and transform isothermally.

What I claim is:

1. A method of refining the structure of super-plastic zinc base eutectoid or near eutectoid alloys that immediately after quenching to ambient temperature retain the high temperature solid phase state during an incubation period prior to the start of a transformation period wherein the alloy transforms to the eutectoid or partly eutectoid state which includes the steps of:

(a) heating the alloy to above its eutectoid temperature;

(b) rapidly cooling the alloy to a sub-ambient temperature;

(c) holding the alloy at said sub-ambient temperature for a period, and

(d) warming the alloy to at least ambient temperature before said transformation period expires.

2. A method according to claim 1, wherein said warming is begun just before the start of said transformation period.

References Cited UNITED STATES PATENTS 3,340,101 9/1967 Fields et al. l4811.5

RICHARD O. DEAN, Primary Examiner U.S. Cl. X.R. 148125 

